Polarizing laser mirror

ABSTRACT

The surface of a laser mirror is formed with grooves to induce polarization of a laser energy incident therein, the plane of polarization being selective and reproducible. The dimensions and spacing of the grooves are a function of the wavelength of the laser energy.

Inventors Louis J. Denes Monroeville, Pa.;

Patrick C. Ward, South Windsor, Conn. 34,234

May 4, 1970 Nov. 2, 197 1 United Aircraft Corporation East Hartford,Conn.

Appl. No. Filed Patented Assignee POLARIZING LASER MIRROR 5 Claims, 4Drawing Figs.

US. Cl .J 33l/94.5, 350/147 lnt.Cl H0ls 3/08 Field of Search 331/945;350/147, 162

[56] References Cited UNITED STATES PATENTS 3,248,660 4/1966 Fajans331/945 3,443,243 5/1969 Patel 331/945 Primary Examiner-William L. SikesAttorney-Anthony J. Crisco ABSTRACT: The surface of a laser mirror isformed with grooves to induce polarization of a laser energy incidenttherein, the plane of polarization being selective and reproducible. Thedimensions and spacing ofthe grooves are a function ofthe wavelengthofthe laser energy.

POLARIZING LASER MiRRoR BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to lasers polarizing laser mirrors.

2. Description of the Prior Art Lasers have an inherent characteristicof seeking the least lossy transfer mechanism during the process ofconverting energy from a lasing medium to a laser beam; laser beamsusually are polarizedsoon after system startup due to thischaracteristic. The generation of a laser beam does not yield polarizedenergy due to any naturally occurring limitation, rather the inevitableinteraction of the laser radiation with the environmental apparatuswithin the laser cavity causes this result. Polarized laser outputs arenot necessarily undesirable; as a matter of fact, the contrary isusually true. it hasbeen observed, however, that for a given opticalcavity arrangement, the polarization of a laser beam will change withtime due to small variations in alignment, temperature and the like, thedirection of polarization appearing almost random thereby causingproblems in the reproducibility of output beam characteristics andsuggesting that a preselectable polarized output would be much moreuseful and desirable.

in general, the polarization of a laser beam results from theinteraction between the beam and the structure causing the polarizationand may occur either inside or outside ofthe optical cavity, preferablythe former. Where accomplished inside the cavity, the system can beinduced to generate laser energy with all of the electromagnetic energypolarized as it is released by the energized lasing molecules.Alternatively, if the laser beam is produced and the selectivepolarization step taken after the beam has left the cavity, energy nothaving the desired direction of polarization is rejected by theselective mechanism and lost; beam energy is then limited to thosecomponents of the original beam which coincide with the desiredpolarization.

There are three devices commonly used in the art to polarize lasers,namely, Brewster angle windows, wire grids and diffraction gratings;each of these has its advantages and disadvantages. Brewster anglewindows are used very frequently and consist simply of a sheet ofmaterial of variable transparency with respect to energy of a givenwavelength, the degree of transparency for the horizontal and verticalcomponents of polarization of the energy being a function of the angleof incidence of the energy upon the sheet. This phenomenon isillustrated in the graph shown on page 511 of Jenkins, F. A. White, H.F. Fundamental of Optics, McGraw- Hill, 1957, 3rd Ed. There is oneangle, the Brewster angle, at which the reflectance for the incominglight for one direction of polarization is zero, i.e., thetransmissivity is maximum. The magnitude of this angle depends uponthe-index of refraction of the window material and the wavelength of theincoming radiation. To induce polarization a window is placed in anoptical cavity at the Brewster angle, thereby allowing the describedselective loss mechanism to preferentially transmit light of aparticular polarization and partially reject light of the nonselectedpolarization. After the light in the optical cavity has oscillated forseveral paths, perhaps three or four, the preferred polarizationdirection becomes dominant Within the cavity and the laser willoscillate in this preferred polarization.

in a typical Brewster angle window arrangement, about four to sixpercent of the energy in the nonpreferred direction is re jected by thewindow in the form of reflection while the energy polarized in theacceptable window direction passes essentially uninhibited. In a laserapplication, there are only several passes through the window beforecomplete polarization occurs and from an efficiency standpoint Brewsterangle win dows are acceptable. By way of comparison, in the polarizationof ordinary white light, approximately successive windows at theBrewster angle would be required before sufficient polarization occurs;this is undesirable and inefficient since there would be a rejection ofapproximately one half of the light which was passing through the windowsystem.

and more particularly to Although windows set at the Brewster angle areefficient and effective at laser cavity polarization, the introductionof such windows into the optical cavity of some gas lasers causes otherproblems. Since the window is not a mirror, the lasing medium, togetherwith the associated Brewster angle windows, must be placed within anoptical cavity (generally two mirrors) in order to form an overall lasersystem. Often such an arrangement results in a region of atmospheric airbetween each window and the nearby cavity mirror. The presence of air isundesirable because variations in the characteristics of the air due tochanges in temperature, pressure, humidity, etc., introduce possiblevariationsin the characteristics of the laser. Further, since opticalcavities require precise alignment of the mirrors and since anyreflecting or transmitting surfaces are possible loss mechanisms, theplacement of any windows between the mirrors and the laser medium isgenerally undesirable; therefore, the mirrors are often fasteneddirectly to the-lasing medium through a bellows attachment. The bellowsserves two purposes, namely, the sealing of the region between thewindows and the mirrors from the surrounding environment, and theallowing of the mirrors to be moved, within certain limits, sufiicientto permit proper alignment of the optical cavity.

To avoid the shortcomings of Brewster angle windows, wire polarizershave been used successfully in lasers; such polarizers can becategorized into large wire and small wire devices. The large wirepolarizer has a wire diameter greater than the wavelength of theincident laser energy. The large wire device specularly reflects thelight polarized in the direction parallel to the length of the wire,scattering much of this energy out of the cavity and inducing the laserto assume a condition having a polarization in the direction normal tothe length of the wire. This selectively occurs quickly and relativelylittle of the total laser energy is lost from the laser cavity in theprocess. However, large diameter wires have a sufficiently large (one ortwo wavelengths) diameter that they are resolva ble by the laser energypolarized orthogonal to the length of the wire. As a result, the wirespartially reflect (thereby removing energy of the desired polarizationfrom the cavity) and also absorb beam energy, becoming heated. A smallwire polarizer is one having a wire diameter less than the wavelength ofthe laser incident energy. It has been found that such polarizers avoidthe described heating problem, however, such devices are extremelyfragile, and difficult to produce and handle. Overall, the wirepolarizer is difficult to align, unable to withstand high-fluxdensities, easily distorted, fragile, and perhaps most importantly insome applications, the wire polarizer has the very great disadvantage ofproviding an output beam with no guarantee of reproducibility of theplane of polarization.

A diffraction grating can be a very suitable polarizing device inaddition to performing its primary function as a wavelength selector.The diffraction grating allows the lasing line of interest to beselectively emitted and to selectively preclude other close but unwantedlasing lines. These gratings, how ever, have disadvantages; they areexpensive and their reflectivity is poor, being more lossy than any ofthe alternate schemes for polarization already discussed. Thefunctioning of a grating in polarizing light is similar to the wiredevice although as a practical matter the grating appears as a fullyreflecting mirror forming one end of the optical cavity. Obviously, thegrating avoids any of the complications inherent in the Brewster anglewindow and wire device arrangements, since each involve introducingadditional components into the laser cavity. Diffraction gratings madeof metal are relatively expensive since they are individually scribed, aprocess requiring elaborate and precise equipment. Although replicadiffraction gratings can be made at a much lesser price, they aretypically formed of plastics unable to withstand the high optical fluxesinherent in the laser oscillators and therefore cannot be used in laserapplications. in operation there is uneven heating in the plasticreplica diffraction grating resulting substantially from the inabilityto adequately cool the substrate plastic.

FIG. 1 is a sectioned, front elevation view of a wire polarizer deviceknown to the prior art;

FIG. 2 is a schematized, side elevation view of a gas laser device,known to the prior art, incorporating a wire polarizer device;

FIG. 3 is a front elevation view of a laser mirror having scribedgrooves in accordance with the present invention; and

FIG. 4 is a broken away sectional view along line 44 of FIG. 3.

SUMMARY OF THE INVENTION A principal object of the present invention isto selectively induce a predetermined direction of polarization in thelaser energy in an oscillator cavity thereby providing polarized output.

According to the present invention, the surface of a laser oscillatormirror is scribed with grooves having dimensions related to thewavelength of the laser energy produced in the cavity, the grooves beingpositioned to introduce into the cavity a reflection loss mechanismhaving a preselected direction, said loss mechanism dominating all otheroptical losses in the cavity and being of sufiicient magnitude to inducethe cavity to produce laser energy completely polarized in the directiondetermined by the scribed grooves.

An advantage of the present invention is the ability to polarize laserradiation into a preselectable plane of polarization. Further, thismethod is considered desirable due to the simplicity of the productionmethod. The polarizing surface is easily controlled when the groovemarkings are formed upon it, and the system is relatively insensitive topower flux incident upon the grooves during operation. This inventioncombines the simplicity of the wire polarizer plus the effectiveness ofa diffraction grating to provide a superior system for causing selectivepolarization of a laser beam as it is being generated in an oscillator.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of a preferred embodiment thereof, as illustratedin the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT Polarization devices of the typeto be described herein have application in various lasers havingdifferent wavelengths. While it will become apparent that the detaileddimensions of these polarizers will necessarily vary according to thewavelength of the laser energy, the mirrors described are with referenceto a carbon dioxide gas laser operating at a wavelength of 10.6 microns;this association is made for convenience and the description of thepreferred embodiment and should not be considered as limiting the scopeof the invention in any manner.

A typical wire polarizer of the type known in the prior art is shown inFIG. 1. The complete device consists of a thin circular ring or disk 12having a inside diameter D sufficiently large to pass the desireddiameter of the laser beam and having an outside diameter at leastsufficient to give reasonable structural integrity to the disk. Attachedto the ring 12 is a plurality of fine wires 14, the wires of diameter d,arranged in parallel alignment at a separation distance s, each wirehaving each of its ends attached permanently to the ring 12. Devicessuch as the wire polarizer 10 have been used in carbon dioxide lasersystems to produce a polarized output beam, however, this type devicehas been found to be relatively inefficient. For example, when threewires arranged in the manner described on a disk having an insidediameter D of approximately 5,000 microns, a spacing, s, between thewires of 1,000 microns and a wire diameter d of 25 microns wereintroduced into the laser system shown in FIG. 2, the wire polarizer 10reduced the output-beam from 6 to 3 watts. The laser system 30 shown inFIG. 2 includes a pair of mirrors 32, 33, the mirror 33 having anaperture 34 of about 2,800 microns and serving as a coupling mirrorthrough which the laser output radiation 35 is trans-' mitted. Themirrors 32, 33 are approximately 2 lficentimeters in diameter and arespaced about 50 centimeters apart from each other. Energy input to thelaser is from an electric discharge 36 which is maintained betweenelectrodes 38, 40. A pair of bellows 42, 44 connect a laser cavityenclosure 45 to the support apparatus of the mirrors 32, 33, and allowalignment of the mirrors through manipulation of control pins 46, 48.The output mirror 33 is edge cooled to ensure thermal stability. Thenontransmitting mirror 32 is a curved substrate of copper, coated withgold.

The phenomenon of metallic reflectivity is due to currents induced inthe metal by incident electromagnetic waves and these currents give riseto a reflected wave. If a plane polarized beam of electromagneticradiation is normally incident on a metal, the induced current will havethe same frequency as the incident wave and the direction of the currentwill lie in the same plane or direction as the electric vector of thepolarized beam. This is the case of ordinary metallic reflection.

A case of special interest occurs if the size of the metal reflectorbecomes comparable with or even appreciably less than the wavelength ofthe incident radiation; the reflectivity (more properly, the effectivescattering ability) depends very strongly on the relative size of thereflector in comparison with the wavelength of the incident radiation.This dependence of reflectivity on the size of the reflector isdeveloped in a theoretical analysis of the scattering of radiation fromsmall metallic spheres in Chapter 13 of Born, M. and Wolf, E.,Principles of Optics, Pergamon Press, 1965, 3rd Ed., in which thepercent reflectivity of the metal spheres is shown to decreasedrastically with decreasing size. A convenient physical interpretationof this effect is to consider that the induced currents set up in areflector by the incident electric wave have difficulty establishingthemselves in a metal whose dimensions are appreciably smaller than thewavelength of the incident radiation. Therefore, the reflectance of ametal is reduced if the material dimensions are very small.

In terms of a practical polarizer consisting of thin metal strips (orwires), currents are established by the incident radiation, thedirection of the currents lying in the plane of the electric vectors ofthe incident wave. For electric vectors that are parallel to the longdimension of the wires, the induced currents are easily established inthe wires since the wires are easily established in the wires since thewires are many thousands of wavelengths long, thus the wires for thispolarization have high reflectivities. Altemately, for electric vectorsof the incident wave that are in the orthogonal direction, the inducedcurrent and the associated reflected radiation are much less, due to thethinness of the wire. Therefore, radiation with a polarization such thatthe electric vector is orthogonal to the long dimension of the wires isreflected less efficiently.

The polarization mechanism of a wire device is straightforward; laserenergy having a polarization vector parallel to the direction of thewire is specularly reflected by the wires and much of this specularlyreflected energy leaves the optical cavity. Laser energy having apolarization vector normal to the long dimension of the wires(senkrecht) undergoes less reflection from the wires since the wirediameter is approximately twice the wavelength of the incidentradiation. This selective reflection by the wires induces the laser toassume the least lossy type oscillation and therefore a polarized beamwith the polarization vector normal to the long direction of the wires.During the use of the wire device shown in FIG. 1 in the laser deviceshown in FIG. 2, it was found that a sufficient amount of the10.6-micron carbon dioxide laser energy was interacting with the wires14 to cause the wires to heat. The heating resulted in both a distortionof the wires and an overall net reduction in the laser output power. Toavoid these problems, attempts were made to reduce the diameter of thewires 14 to less than the wavelength (10.6 microns) of the incidentradiation. It was found however, that the resulting polarizer had wiresthat were extremely thin and difficult to handle and align.

In accordance with the present invention, a scribed polarizing mirror 56shown in FIG. 3 has been developed. The surface 58 of mirror 56 hasscribed grooves 60 shown in more detail in FIG. 4 for the case of a 10.6micron laser in which the grooves 60 have a width w of approximately .4microns and a depth I of approximately 25 microns. This mirror may beused, inter alia, in a laser system of the type illustrated in FIG. 2 bysubstituting it for the mirror 32 and removing the wire polarizer 10.

The polarization mechanism of the grooved surface is different than thatpreviously described for a wire device. The grooved surfacepreferentially reflects radiation polarized in a direction perpendicularto the groove until the laser has oscillated a sufficient number oftimes to assume the least lossy oscillation and completely assume apolarization parallel to the grooves. At this point the laser oscillatesas though the scribed surface were smooth.

When the device shown in FIG. 3 has been used in an operating lasercavity very similar to that shown in FIG. 2, the laser power output wasincreased by a factor of two over the same cavity run with a wirepolarizer. The mirror used had two parallel lines scribed on itssurface, however, more lines are possible; the greater the number oflines the greater the pertubation introduced into the lasing medium.Similarly, a lesser number of lines might be used although, if the totalgrooving is of insufficient length, there may not be enough selectiveloss to cause the desired polarization. Specifically, in the design ofthe grooves, the total length should be at least twice the diameter ofthe burn spot (diameter of the lasing mode at the surface of the groovedmirror) on the mirror surface having grooves scribed thereon. Minimumspacing between any two of the parallel surface grooves should be manywavelengths; a separation of one hundred wavelengths has been foundsatisfactory. Also, the width w should be less than approximatelyone-quarter of the wavelength of the laser radiation, and preferablymuch less. The narrower the groove, the greater the number of groovesthat will be needed to cause the desired polarization; however, it ispreferable to have several narrow lines rather than a fewer number ofwider lines. The depth 1 of the scribed groove should be more than awavelength and preferably more than one wavelength (of the order of aquarter of a wavelength), the mirror surface including the scribed ruleswill act as a specular reflector and may not have a polarizing effect onthe laser cavity radiation. It should be apparent that, as thewavelength of the laser energy increases, the precision and difficultyrequired in the preparation of the polarization mirror is decreased.

Although the invention has been shown and described with respect to butone preferred embodiment thereof, it should be obvious to those skilledin the art that various changes and omissions in the form and detailthereof may be made therein without departing from the spirit and thescope of the invention.

Having thus described a typical embodiment of our invention, that whichwe claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An optical cavity adapted to contain a suitably energized lasermedium to provide an output beam of laser energy having a desiredwavelength, said cavity comprising:

a pair of mirrors having a spacing therebetween which causes the opticalcavity to be resonant at said desired wavelength;

a first mirror of said pair of mirrors being a coupling mirror whichtransmits laser energy from said cavity; and

a second mirror of said pair of mirrors having grooves which are lessthan said desired wavelength in width and greater than said desiredwavelength in depth, said grooves being mutually parallel to one anotherand separated by a distance of at least times said desired wavelength,said second mirror preferentially supporting the generation of laserenergy which is polarized along one of two mutually orthogonaldirections with one of said orthogonal directions parallel to saidgrooves, the total length of said grooves being sufficient to cause saidoutput beam to become substantially completely polarized in a preferreddirection.

2. The cavity according to claim 1 wherein each of said grooves has awidth no greater than one-half of said wavelength and a depth of no lessthan two wavelengths.

3. The cavity according to claim 2 wherein each of said grooves has awidth no greater than one-quarter of said wavelength and a depth no lessthan five wavelengths.

4. In a carbon dioxide laser optical cavity providing an output beam oflaser energy and including at least an output coupling mirror and asecond, fully reflecting mirror, said mirrors having a lasing mediumdisposed therebetween and forming a laser oscillator region, theimprovement in said reflecting mirror comprising:

said reflecting mirror having a plurality of parallel grooves formed inthe surface thereof, said grooves having a total cumulative lengthgreater than twice the diameter of the lasing mode of said output beam,a width of approximately 4 microns, a depth of approximately 25 micronsand minimum spacing between parallel grooves of approximately 1000microns.

5. An optical cavity according to claim 1 wherein the total length ofsaid grooves is at least twice the diameter of the lasing mode of saidlaser beam at the surface of said second mirror.

2. The cavity according to claim 1 wherein each of said grooves has awidth no greater than one-half of said wavelength and a depth of no lessthan two wavelengths.
 3. The cavity according to claim 2 wherein each ofsaid grooves has a width no greater than one-quarter of said wavelengthand a depth no less than five wavelengths.
 4. In a carbon dioxide laseroptical cavity providing an output beam of laser energy and including atleast an output coupling mirror and a second, fully reflecting mirror,said mirrors having a lasing medium disposed therebetween and forming alaser oscillator region, the improvement in said reflecting mirrorcomprising: said reflecting mirror having a plurality of parallelgrooves formed in the surface thereof, said grooves having a totalcumulative length greater than twice the diameter of the lasing mode ofsaid output beam, a width of approximately 4 microns, a depth ofapproximately 25 microns and minimum spacing between parallel grooves ofapproximately 1000 microns.
 5. An optical cavity according to claim 1wherein the total length of said grooves is at least twice the diameterof the lasing mode of said laser beam at the surface of said secondmirror.